AArch64: Compressed pointer decoding optimization when base is zero.

Currently decoding a compressed pointer with zero base will generate
the following code:
add x0, xzr, x0, lsl #3

This patch will optimize it to:
lsl x0, x0, #3

Here is a jmh benchmark to decode a compressed pointer:
public class UncompressPointer {
    private static final int field0 = 1000000;
    private int field1;

    static class A {
        B b;

        A(B b) {
            this.b = b;
        }
    }

    static class B {
        int field;

        B(int i) {
            this.field = i;
        }
    }

    static A[] arr;

    static {
        arr = new A[field0];
        for (int i = 0; i < field0; i++)
           arr[i] = new A(new B(i));
    }

    public static int func() {
        int result = 0;
        for (int i = 0; i < field0; i++) {
            result += arr[i].b.field;
        }
        return result;
    }

    @benchmark
    public void jmhUncompressPointer() {
        field1 = func();
    }
}
And the performance results:
                         Score      Error    Units
Without this patch     21598.534 ? 2188.242  us/op
With this patch        19322.791 ? 1219.022  us/op

Change-Id: I5af6799eb82470d5598f5991e9210f99874f458a

compiler/mx.compiler/suite.py

@@ -668,6 +668,7 @@
       "dependencies" : [
         "org.graalvm.compiler.asm.aarch64",
         "org.graalvm.compiler.hotspot.test",
+        "org.graalvm.compiler.hotspot.aarch64",
       ],
       "annotationProcessors" : ["GRAAL_NODEINFO_PROCESSOR"],
       "checkstyle" : "org.graalvm.compiler.graph",

compiler/src/org.graalvm.compiler.hotspot.aarch64.test/src/org/graalvm/compiler/hotspot/aarch64/test/AArch64UncompressPointerTest.java

@@ -0,0 +1,125 @@
+/*
+ * Copyright (c) 2019, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2019, Arm Limited and affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package org.graalvm.compiler.hotspot.aarch64.test;
+
+import jdk.vm.ci.aarch64.AArch64;
+import jdk.vm.ci.code.Register;
+import jdk.vm.ci.code.TargetDescription;
+import jdk.vm.ci.runtime.JVMCI;
+import org.graalvm.compiler.asm.aarch64.AArch64Assembler;
+import org.graalvm.compiler.asm.aarch64.AArch64MacroAssembler;
+import org.graalvm.compiler.core.test.GraalCompilerTest;
+import org.graalvm.compiler.hotspot.aarch64.AArch64HotSpotMove;
+import org.junit.Before;
+import org.junit.Test;
+
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assume.assumeTrue;
+
+public class AArch64UncompressPointerTest extends GraalCompilerTest {
+
+    private AArch64MacroAssembler masm1;
+    private AArch64MacroAssembler masm2;
+    private Register input;
+    private Register result;
+
+    @Before
+    public void checkAArch64() {
+        assumeTrue("skipping AArch64 specific test", JVMCI.getRuntime().getHostJVMCIBackend().getTarget().arch instanceof AArch64);
+    }
+
+    @Before
+    public void setupEnvironment() {
+        TargetDescription target = JVMCI.getRuntime().getHostJVMCIBackend().getTarget();
+        masm1 = new AArch64MacroAssembler(target);
+        masm2 = new AArch64MacroAssembler(target);
+        input = AArch64.r10;
+        result = AArch64.r11;
+    }
+
+    private void emitUncompressPointer(Register base, int shift) {
+        AArch64HotSpotMove.UncompressPointer.emitUncompressCode(masm2, input, result, base, shift, true);
+    }
+
+    private void compareAssembly() {
+        byte[] expected = masm1.close(false);
+        byte[] actual = masm2.close(false);
+        assertArrayEquals(expected, actual);
+    }
+
+    @Test
+    public void testUncompressPointerWithBase() {
+        Register base = AArch64.r12;
+        int shift = 3;
+        masm1.add(64, result, base, input, AArch64Assembler.ShiftType.LSL, shift);
+        emitUncompressPointer(base, shift);
+        compareAssembly();
+    }
+
+    @Test
+    public void testUncompressPointerWithZeroBase() {
+        int shift = 3;
+        masm1.shl(64, result, input, shift);
+        emitUncompressPointer(null, shift);
+        compareAssembly();
+    }
+
+    @Test
+    public void testUncompressPointerWithZeroBaseAndShift() {
+        masm1.or(64, result, AArch64.zr, input);
+        emitUncompressPointer(null, 0);
+        compareAssembly();
+    }
+
+    static class A {
+        String str;
+
+        A(String str) {
+            this.str = str;
+        }
+    }
+
+    public static String getObjectField(A a) {
+        return a.str;
+    }
+
+    @Test
+    public void testGetObjectField() {
+        test("getObjectField", new A("asfghjkjhgfd"));
+    }
+
+    static String[] strings = {"asf", "egfda", "fsdasere", "eqwred", "fgdadgtre", "qwrrtreety"};
+
+    public static String getArrayMember(int index) {
+        return strings[index];
+    }
+
+    @Test
+    public void testGetArrayMember() {
+        test("getArrayMember", 4);
+    }
+}

compiler/src/org.graalvm.compiler.hotspot.aarch64/src/org/graalvm/compiler/hotspot/aarch64/AArch64HotSpotMove.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2013, 2018, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2013, 2019, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -194,9 +194,19 @@ public void emitCode(CompilationResultBuilder crb, AArch64MacroAssembler masm) {
         }
 
         public static void emitUncompressCode(AArch64MacroAssembler masm, Register inputRegister, Register resReg, Register baseReg, int shift, boolean nonNull) {
+            // result = ptr << shift
+            if (baseReg == null) {
+                if (shift != 0) {
+                    masm.shl(64, resReg, inputRegister, shift);
+                } else if (!resReg.equals(inputRegister)) {
+                    masm.movx(resReg, inputRegister);
+                }
+                return;
+            }
+
             // result = base + (ptr << shift)
-            if (nonNull || baseReg == null) {
-                masm.add(64, resReg, baseReg == null ? zr : baseReg, inputRegister, AArch64Assembler.ShiftType.LSL, shift);
+            if (nonNull) {
+                masm.add(64, resReg, baseReg, inputRegister, AArch64Assembler.ShiftType.LSL, shift);
             } else {
                 // if ptr is null it has to be null after decompression
                 Label done = new Label();